Eukaryotic Algae: Reproduction and Life Cycles

• Eukaryotic algae encompasses diverse organisms with intracellular organization similar to higher green plants.
• These algae belong to Kingdom Protista.
• They lack vascular tissues and are not differentiated into roots, stems, or leaves.
• They contain chlorophyll and accessory pigments like cyanobacteria.
• Eukaryotic algae under the class Rhodophyceae exhibits pink/red pigmentation from phycoerythrin.
• Pyrrophyta consist of dinoflagellates.
• Chrysophyta consist of diatoms.
• Phaeophyta consist of true brown algae.
• Euglenophyta and Chlorophyta consist of green algae.

Algal Reproduction and Life Cycles

• Algae reproduce vegetatively, asexually (spores), or sexually (gametes).
• Vegetative and asexual reproduction enable rapid production of individuals with stable genotypes.
• Sexual reproduction allows for variation but is more costly due to wasted gametes.
• Algae life cycles are classified as haplontic/zygotic, diplontic/gametic, or diplohaplontic.
• Haplontic/zygotic life cycle: Predominantly vegetative, haploid, meiosis occurs during zygote germination.
• Diplontic/gametic life cycle: Characterized by a single, predominant vegetative diploid phase; meiosis yields haploid gametes.
• Diplohaplontic life cycle: Alternation of generations between haploid gametophyte and diploid sporophyte phases.
• Gametophytes produce gametes via mitosis; sporophytes create spores via meiosis.
• A generalized cycle is seen most members of Florideophycidae.
• Red algae exhibit a triphasic life cycle with gametophyte (n), carposporophyte (2n), and tetrasporophyte (2n).

Division Rhodophyta (class Rhodopyceae)

• Red algae are different from other pigmented algae due to:
  • Complete absence of flagella and centrioles.
  • Absence of sexuality in some species.
  • Lack of plasmodesmata.
  • Presence of pit connections.
• Thylakoids are unstacked and grouped into phycobilisomes, similar to cyanobacteria.
• Subclass Bangiophycidae:
  • Unicellular forms, uninucleated cells.
  • Absence of pit connections.
  • Absence of true sexual reproduction in most members.
• Subclass Florideophycidae:
  • Unicellular forms, uninucleated cells
  • Absence of pit connections
  • Absence of true sexual reproduction in some members.

Classification of Division Rhodophyta

• Class Rhodophyceae includes:
  • Subclass Bangiophycidae:
    • Order Porphyridiales
      • Family Porphyridiaceae (Porphyridium cruentum)
    • Order Bangiales
      • Family Bangiaceae (Porphyra sp.)
    • Order Compsopogonales
      • Family Comsopogonaceae (Composopogon caeruleus)
  • Subclass Florideophycidae:
    • Order Nemaliales
      • Family Galaxauraceae (Galaxaura sp.)
    • Order Gracilariales
      • Family Gracilariaceae (Gracilaria sp.)
    • Order Gigartinales
      • Family Solieriaceae (Eucheuma sp.)
    • Order Ceramiales
      • Family Rhodomelaceae (Polysiphonia sp.)

Division Pyrrophyta – Dinoflagellates

• Dinoflagellates are unicellular protists.
• They have a biflagellated nature of the vegetative cells.
• The flagella differ in form, orientation, and movement type.
• Movement of flagella could be inserted anteriorly or laterally.
• Vegetative cells are enclosed by a cellulose-rich cell covering.
• Members belong to Pyrrhophyta and classes Desmophyceae and Dinophycea.
  • Desmophyceae have apically inserted flagella.
  • Dinophycea have laterally inserted flagella:
    • One is at the cingulum.
    • One is at the sulcus.
• Reproduction is commonly asexual via binary fission.
• Sexual reproduction occurs in some.
• Bloom-forming species can encyst under unfavorable conditions to form cysts that differ from the vegetative cell and are a taxonomic character.
• In summer, dinoflagellates can reproduce rapidly, causing "red tide".
• Marine life is affected due to neurotoxins.
• Representative species include Pyrodinium bahamense, Prorocentrum, and Ceratium.

Division Chrysophyta - Diatoms

• Diatoms belong to Chrysophyta and class Bacillariophyceae, they have cell walls called frustules mostly made of silica.
• Frustules consist of two overlapping halves fitting like a petri dish.
• Cell wall structures and symmetry are taxonomic characters.
• Diatoms are grouped into centric (radial symmetry) and pennate (bilateral symmetry) types.
• Reproduction is usually by longitudinal division, resulting in successive cell size reduction.
• Auxospores restore maximum cell size, and auxospore formation is a sexual process.
• Gametes fuse via isogamy, anisogamy, or oogamy to form auxospores.
• Representative species include Cyclotella, Synedra, and Navicula.

Division Phaeophyta

• Brown algae is the largest group, growing in shallow waters and on rocky shores.
• They are characterized by xanthophylls-fucoxanthin.
• They contain carotenoids and phaeophycean tannins.
• Their color varies from brown-yellow, green-brown, to dark brown.
• Phaeophytes are the largest group among chromists.
• They contain chlorophyll c and fucoxanthin pigments.
• True brown algae lack unicellular or colonial forms.
• They are grouped into one class (Phaeophyceae) and three subclasses: Isogeneratae, Heterogenerate, and Cyclosporae.
• Subclasses are based on the type of alternation of generations.
  • Isogeneratae and Heterogeneratae possess diplohaplontic life cycles.
    • Isogeneratae feature isomorphic alternation.
    • Heterogeneratae demonstrate heteromorphic alternation.
  • Cyclosporae have a diplontic life cycle, lacking alternation of generations.
• Kelps, large brown algae seaweeds, form "underwater forests."
• Representative species include:
  • Fucus (rockweed): Macro-algae found on rocky seacoasts and salt marshes, used for alginate production.
  • Sargassum: Widely distributed genus, especially in the tropic, with the same life cycle as Fucus.
  • Ectocarpus: Simplest branched filament.
• Sporophyte (2n thallus) and gametophyte (n) are morphologically alike.
• Laminaria: Complex forms and tissue.
  • Kelp grows up to 5 m (or more) in length on rocky coasts of temperate/polar seas.
  • Demonstrates heteromorphic alternation of generation.
• Hydroclathrus: Net-like brown alga in the tropic.
  • Same alternation of generations as Laminaria.
  • Produces plurilocular sporangia on blade margins.
• Padina jamaicensis: leaf-like white scroll algae attached to rocks in shallow reefs.

Green Algae

• Green algae contain chlorophylls a and b that store starch inside double membraned plastids.
• Chlorophytes and Charophytes are two distinct groups resembling higher plants.
• Like other algae, they can be unicellular or colonial.

Division Chlorophyta

• Includes scaly green flagellates (Prasinophytes; Pyramimonas and Tetraselmis), Ulvophyceae (Ulva and Acetabularia), Chlorophyceae (Chlamydomonas and Volvox), and Trebouxiophyceae (Chlorella).
• Mostly live in marine water.

Chlorophyceae

• Species are distinguished by clear green color due to chlorophyll.
• Orders include:
  • Volvocales, Chaetophorales, & Chlorococcales: Possess an offset flagellar arrangement (1 o'clock-7 o'clock); constitute over half of all chlorophyceans.
  • Chlorellales: Opposed flagellae (12 o'clock-6 o'clock), some only have vestigial flagellae; similarities with Chlorococcales make distinctions challenging.
  • Oedogoniales: Complex multiflagellate crown on swimming spores, filamentous, oogamous, and contain net-like chloroplasts.

Division Streptophyta

• Includes all embryophyte plants (land plants) and freshwater green algae.
  • Charophyceae
  • Coleochaetophyceae
  • Zygnematophyceae
  • Klebsormidiophyceae
  • Mesostigmatophyceae.

Charophyceae

• Members form macroscopic branching filaments with a long central axis.
• They are punctuated by nodal cells, each with whorl of secondary branches and are coenocytic.
• Nodal cells are coenocytic.
• Stems are long between nodal cells.
• Four orders:
  • Klebsormidiales
  • Zygnematales
  • Coleochaetales
  • Charales.
• Species contain a jacket of cells surrounding the oogonium.
• Flagellate sperm cells have structures similar to bryophytes and ferns.
• Charophyte oospores have resistant cell walls.